1. Field of the Invention
The present invention relates to a magnetic type printing process and to a printing machine employing this process. It finds application in data processing, telecommunications, telecopying, etc. . . .
2. Description of the Prior Art
The principle of magnetic printing machines is known. Magnetized areas of very small size (which will hereinafter be referred to as "elementary" areas) may be recorded on a magnetized support or carrier. These areas are commonly grouped in matrix form, in blocks of 5.times.7 or 7.times.9 elements, for example. They render it possible to outline latent magnetic characters having the form of printing characters. A pulverulent product containing ferromagnetic particles is deposited on this carrier and the particles are attracted by and affixed on the magnetized areas to thus display the characters recorded. The carrier coated with the developer product may then be applied onto the document to be printed in order to transfer the recorded characters onto this document. An erasing device erases the characters recorded which renders the carrier again available for receiving fresh symbols.
The method of recording magnetized areas on a magnetic carrier is well known. In order to magnetize a material, the material is first exposed to a magnetic field whose intensity is sufficient for the material to be saturated, that is, for the magnetic induction to have a limiting value. The magnetic field is then cancelled. A magnetic induction differing from a nil value, which is referred to as remanent induction, the remains within the material.
Magnetic carriers are commonly of two kinds:
1. longitudinal magnetization carriers, wherein the direction of magnetic induction is parallel to the carrier surface; and
2. perpendicular magnetization carriers, wherein the direction of magnetic induction is perpendicular to the surface of the carrier.
A device for recording characters is thus formed by coordinating a magnetic carrier of the longitudinally or perpendicular magnetization type with a plurality of recording transducers. Each transducer commonly consists of an electromagnet whose air gap is a very narrow slot not exceeding a few microns, so that the lines of magnetic induction in air are concentrated. The carrier utilized for magnetic printers may take a variety of forms, such as, for example, drum, tape, etc. . . .
Means of this type can for instance comprise a carrier produced from a material of high magnetic permeability, such as iron or mild steel. The carrier is coated with a layer of magnetic material. The perpendicular magnetization of this carrier is produced by means of a recording transducer comprising a magnetic core on which a coil is wound. This device is formed in such manner as to have a recording pole and a flux closure pole whose cross-section is greater than that of the recording pole.
In respect of the means for depositing a developer product on a magnetic carrier, this may take many forms such as, for example, an air jet device, a magnetic conveyor or a vibrating plate.
In the printing machines of this kind, the developer product employed consists of a pulverulent ferromagnetic pigment of very small grain size, mixed with a coating product. The magnetic grains of the developer product have much smaller dimensions that the dimensions of each elementary magnetized area (for example, a few microns as opposed to some hundred microns). This product must fulfill very stringent conditions. It must not only adhere to the magnetized areas of the recording element, but must equally be transferable on to the receiving paper sheet with which it is placed in contact. It must be able to meet freely at a relatively low temperature, to avoid the risks of burning or carbonizing the paper, but its melting point must not be too low, so as to avoid the appearance of a softening action which would inhibit the transfer on to the paper. Moreover, if appropriate, this product must accept a coloring agent intended to increase the contrast between the paper background and the printed symbols. Finally, it is essential that the pulverulent product should be formed by relatively fine solid particles so that it produces images of satisfactory quality on the paper. However, when they are melted, these particles must neither spread over the paper, nor diffuse within the same to avoid images which would appear blurred.
It should be appreciated that the production of an appropriate developer product is both an essential matter and a problem difficult to resolve, and it should certainly be appreciated that none of the products now utilized give complete satisfaction.